Denon TU-850
Three moving-coil meters on a tuner face - that's the tell. Denon built this one for people who wanted to watch the signal, not just hear it.
Overview
The TU-850 is the tuner half of Denon's 850 Series, a separates pairing that also included the PMA-850 integrated amplifier. Denon's own German-language literature from the period calls the two "zwei Top-Modelle einer neuen DENON-Bausteinlinie" - two top models of a new Denon component line. Most tuners of the era gave you one meter. The TU-850 has three moving-coil instruments: one for channel center, one for field strength, and one for multipath. Multipath is the reflections and phase disturbances that wreck FM reception in cities long before they show up as audible noise. A rotary METER SELECTOR also lets the meters display the audio output level, or, in the "power out" position, the output power of the matching amplifier's output stages. That last trick only makes sense if you own the PMA-850.
The tuner has a switchable IF bandwidth, "wide" and "narrow," which was an expensive design choice. Wide bandwidth preserves the full quality of a strong local station. Narrow bandwidth pulls apart stations crowded close together on the dial. Denon's literature says that in dense broadcast areas the sidebands of adjacent stations actually overlap, and a fixed-bandwidth tuner has to compromise somewhere. The TU-850 doesn't. Denon claims that even in stereo, with the wide setting engaged, distortion stays below 0.06%, and channel separation reaches 50 dB.
The demodulator is where Denon put its real engineering effort. Instead of the germanium diodes used in the standard ratio detector of the day, the TU-850 uses DC-biased silicon diodes, along with other refinements to the familiar circuit. According to the manufacturer, the signal-to-noise ratio is 84 dB (IHF), which Denon describes as meaning no audible background noise on a received program, assuming a proper antenna signal. The stereo decoder got similar attention: a refined PLL circuit that Denon claims brings stereo reception to the same noise-free quality as mono. That's marketing language, but stereo FM decoding in the 1970s did add audible hiss.
Denon's US advertising called the 850 Series "perfectionist equipment for serious listeners," quoted 85/85 watts of DC amplification from the matching amplifier, and said the set of separates "defies comparison" - Audio (January 1979, p.148). That's the manufacturer's own copy, not an independent verdict. The front panel carries a High Blend switch to tame stereo noise on weak signals, a muting switch, and a Record Level position that routes a test tone to the tuner's audio outputs to help set recording levels on a tape deck.
Reception and Use
Owners of the 850 Series tend to describe the tuner as the more interesting half of the pair. The three meters are the reason. Multipath indication is rare on consumer tuners, and it turns antenna alignment from guesswork into something you can see. Rotate the antenna, watch the multipath needle drop, hear the distortion fall away. That's a workflow borrowed from broadcast engineering.
The TU-850 is not a digital-synthesizer tuner. Tuning is done by hand, and the accuracy of reception depends on the operator's patience and the quality of the antenna feed. Denon's own literature assumes a proper HiFi-grade antenna signal for the 84 dB noise figure to mean anything. In weak-signal conditions, the narrow IF setting is the one that earns its keep. In strong-signal conditions, the wide setting is the one you want, and the difference is not subtle on a good broadcast.
Documentation for the TU-850 survives in the form of the original Denon/Bolex German-language sales literature, which lists the model alongside the PMA-850 and carries a print code of 6/78 — suggesting the line was being distributed in Germany by mid-1978. The same document is signed by Bolex GmbH of Ismaning, near Munich, as a member of the German High Fidelity Institute. That's a distribution detail rather than a design one, but it confirms the 850 Series was a coordinated European launch, not a Japan-only curiosity.
Collectibility
The TU-850 turns up less often than Denon's later, more numerous tuners, and the three-meter front panel is why collectors notice it when it does. Complete 850 Series systems, tuner plus PMA-850, are the prize, since the "power out" meter function is only useful with the matching amplifier. As with any 1970s tuner, the condition of the tuning capacitor and the electrolytic capacitors in the power supply decides whether a unit is a daily driver or a restoration project. The service literature is available, and the circuit is conventional enough that a competent technician can align it.
It is not a high-dollar collectible, and it is not rare in the way that flagship tuners are rare. It is, instead, a well-documented example of a specific moment: the late 1970s, when Denon was building a coordinated separates line with genuine broadcast-adjacent engineering and selling it on the strength of what the meters could show you. For a collector who wants a tuner that rewards attention rather than one that just works in the background, that's the pitch.
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